With the rapid development of solid-state lighting technology, high-brightness light emitting diodes (LEDs) have been widely used in many lighting equipment, such as street lightings, LED-backlit LCD displays, etc. High-power LED lamps typically consist of multiple parallel-connected LED strings each having one or more series-connected low-power LEDs. Due to considerable tolerance in the voltage-current characteristics of LEDs and the linear relationship between the luminous output and the LED current, it is often important to operate the LED strings with the same current so as to ensure uniform luminous output from the LED strings.
Current-balancing techniques for balancing current through different LED strings in LED circuits can be broadly classified as passive techniques and active techniques. Generally, passive techniques are commonly based on using passive circuit components, such as resistors, capacitors, inductors, and transformers, to balance the string currents; while active techniques are commonly based on using active circuit devices to control the string currents. In terms of circuit complexity, passive techniques are generally simpler compared with active techniques as they normally require only one main switching network. Therefore, passive techniques are more preferable over active techniques in some applications.
Nevertheless, existing current balancing techniques are not without problems. For example, in one technique based on inserting a resistor in series with each string, the resistors may readily introduce energy loss in the circuit and this type of technique is often less suitable for applications requiring dimming function. In some other techniques, the reactive power and circulating energy in the LED driver may be substantial and hence not desirable. In yet some other techniques, such as those using transformers, the need of an additional energy-recycling or feedback network as well as the cost and physical size of the circuits are problematic.